1. Field of the Invention
The present invention relates to a high-power semiconductor device having a bipolar-MOS composite element and, more particularly, to a semiconductor device having a bipolar-MOS composite element pellet suitable for a pressure contacted structure.
2. Description of the Related Art
Conventional high-power semiconductor devices are available as devices for performing switching of large currents of several tens of amperes. Of these devices, a device incorporating a bipolar-MOS composite element pellet such as an IGBT (Insulated Gate Bipolar Transistor) is available as a high-power semiconductor device capable of a high-speed operation.
A conventional device of this type is described in detail in High-Power, High-Speed Toshiba IGBT Module, TOSHIBA CORPORATION, September 1987, p4.
As especially indicated by the diagram in the second step in the Assembly flow of FIG. 4 on page 4 in High-Power, High-Speed Toshiba IGBT Module, in the conventional device incorporating a bipolar-MOS composite element pellet, the pellet is electrically connected to external electrodes by wire bonding. Although such a conventional device is designed to conduct large currents, since the bonding wires serve as main current paths, the allowable current is small. If an overcurrent exceeding the allowable current flows in the device for some reason, bonding wires may be fused or may become disconnected from electrodes on the pellet or from external electrodes. Fusing and disconnection of bonding wires are especially dangerous for devices using large currents. Currently, for example, a fuse is arranged independently of a device so that if fusing/disconnection of bonding wires is caused, the fuse is fused to quickly stop the device to prevent explosion of the device.
In addition, such a device tends to generate heat due to switching of large currents. For this reason, degradation or breakage of bonding wires tends to occur especially due to thermal fatigue. This shortens the service life of each bonding wire and hence the service life of the device.
Furthermore, since bonding wires are bonded to electrodes consisting of aluminum, process fluctuation leads to fluctuation in bonding strength. This means that if bonding strength is decreased, disconnection of bonding wires from electrodes tends to occur.